1. Field of the Invention
This invention relates to a new polyurethane prepolymer and foam compositions and to methods of preparing these materials.
2. Description of the Previously Published Art
Flexible polyurethane foams have been made from prepolymers that have their ends capped with an isocyanate. The preferred commercially used isocyanate for capping is tolylene diisocyanate, TDI, which is also known as toluene diisocyanate. These isocyanate capped prepolymers are reacted with water to form flexible foams. An example of this prepolymer system is described in the Wood et al U.S. Pat. No. 4,137,200.
When utilizing this prepolymer foam making system it is also possible to add a large quantity of fire retardant materials to the aqueous phase. The resulting polyurethane foams are fire resistant because of the composition of the polyurethane and because they contain large amounts of fire retardant materials. Such a system is disclosed in the Marans et al U.S. Pat. No. 4,165,411.
These two patents list many isocyanates other than TDI for capping the prepolymers and included in the list is PAPI a commercially available polymerized form of methylene diphenyl isocyanate, MDI, as defined in U.S. Pat. No. 2,683,730. However, in all of the examples in both of the Wood et al and Marans et al patents the preferentially used isocyanate is tolylene diisocyanate, TDI if a flexible foam is desired since the polymerization of polymeric MDI results in rigid foams.
When flexible polyurethane foams are desired which are made from an isocyanate it has been the understanding that TDI is to be used as the isocyanate since MDI type isocyanates are generally only used for making rigid foams. See, for example, the discussion of an MDI application grid in the Technical Service Report of the Upjohn Polymer Chemicals Company on "Cast Elastomers and RIM Processing: The MDI Bridge" where they point out the entire grid can be formulated with MDI-type isocyanates with the exception of flexible cushioning and some refrigeration applications. In other words, the MDI is not capable, in the opinion of the manufacturer, to be used for flexible foams. Similarly, the "Encyclopedia of Polymer Science and Technology", Vol. 11 (John Wiley and Sons, Inc. 1969) in the section on flexible polyurethane foams at page 538 also shows that MDI is not used in flexible foams with the following statement. "Although various diisocyanates have been used in preparing flexible foams, for all practical purposes, tolylene diisocyanate (TDI) is used almost exclusively. The 80:20 mixture of 2,4- and 2,6- isomers is most commonly used commercially in the United States."
For polyurethane foams made by the prepolymer system where a prepolymer is first formed by capping each end of a polyol with at least a difunctional isocyanate, TDI has been the preferred material to use as the isocyanate, TDI has been the preferred material to use as the isocyanate. One reason for the use of TDI is because it permits excellent control in the initial reaction with the polyol. The isocyanate group adjacent the methyl moiety in TDI is sterically hindered and has a lower reactivity than the isocyanate group in the para position. When making the prepolymer essentially only the isocyanate group in the para position initially reacts with the polyol leaving the isocyanate group in the ortho position to react later when the prepolymer is foamed by mixing with water. Such a controlled, sequential reactivity of the two isocyanate groups was not expected to be possible with MDI since it has two isocyanate groups at symmetrical locations with each one having the same reactivity. Thus one would expect it would be difficult to control the reactivity of the isocyanate groups when making the MDI prepolymer and that uncontrolled chain extension would take place. Similarly, when using a polymeric MDI having a functionality greater than 2.0 to make a prepolymer, one would expect there would be excessive crosslinking because of the extra isocyanate groups.
Although TDI containing foams are the current material of choice for making flexible polyurethane foams from isocyanate capped prepolymers, they do have three disadvantages. One is that in these foams the TDI can form TDA, tolylene diamine, and this material is considered to be a potential carcinogenic material. Thus, for health reasons it would be preferred to use a polyurethane foam which does not contain TDI. Another is that TDI itself is a relatively volatile, low boiling, high vapor pressure monomer that causes problems with the health of workers who are exposed to these vapors in the foam production operations. The third disadvantage is that the TDI containing foams made from the polyols described in the Wood et al U.S. Pat. No. 4,137,200 do not have good hydrolytic stability which is a measure of the resistance to wet aging and a measure that would be predictive of the rate of degradation in a wet environment.
3. Objects of the Invention
It is an object of this invention to produce a flexible polyurethane prepolymer and foam which does not contain TDI, which promotes improved safety to workers, and which has even better compression set and hydrolytic stability.
It is a further object to produce a polyurethane foam from a prepolymer which has as the major source of the isocyanate in the prepolymer a polymeric MDI isocyanate.
It is a further object of the invention to produce a polyurethane prepolymer which has a polymeric MDI as its major isocyanate component.
It is a further object to produce fire retardant polyurethane foams which contain polymeric MDI as the major isocyanate component of the prepolymer and which have good compression set, good hydrolytic stability, lower smoke generation, and which will generate less toxic products on burning.
It is an object to produce polymeric MDI containing flexible foams with compression set values of less than 25% and preferably less than 20%.
These and further objects will become apparent as the description of the invention proceeds.